The rotary output member or rotor of conventional downhole positive displacement motors employed in earth drilling operations are eccentric, by an amount in the order of approximately 5/8 of an inch, with respect to the axis of the drill string and the input member of a bearing assembly to which the rotor must be secured. Accordingly, a drive shaft having universal joints at each end is commonly employed to connect the output member of the motor to the input member of the bearing assembly.
The necessary universal joint arrangement typically employed heretofore is comprised of a pair of tubular members having inter-locking axially extending teeth which transfer of torque between the tubular members while the members are disposed at an angle to one another. A ball and seat arrangement, located in the interior of the universal joint assembly, serves to maintain the tubular members operatively engaged.
The ball and seat and the inter-locking teeth must be kept free of drilling mud for proper operation and optimum life expectancy of the universal joint assemblies. To that end, a flexible sleeve is typically placed over each universal joint assembly so as to seal the joints from the drilling fluid. The sleeve encased universal joints are filled with lubricating fluid under appropriate pressure. This form of sleeve is preferred because it is relatively inexpensive and easily serviceable.
There is a major drawback to the above described universal joint assembly. For whatever reason, there is a tendency for the flexible sleeve to become caught between and torn by the teeth of the tubular universal joint members. This will allow drilling mud to enter the universal joint assembly and, as explained earlier, may result in premature failure of the universal joint assembly. Because of the nature of the conventional joint construction, it is difficult to employ any other form of relatively inexpensive and easily serviceable seal arrangement. There is seen to be a need, therefore, for an alternative form of universal joint assembly which can be used with the conventional sleeve seal arrangement or which will permit the use of other forms of sealing arrangements.
At least in part, the difficulty described above is thought to result from pressure fluctuations of the drilling mud which forces the sleeve into the teeth of the universal joint. Clearly, regardless of the type of seal employed, such pressure fluctuations may adversely affect the operation of the sleeves.